A long-term objective of neurobiology is to describe behaviors of relevance to the human condition such as vocal learning in quantitative terms based on neuronal mechanisms. An excellent model system for vocal learning is the acquisition of avian song. Song learning exhibits a variety of developmental phenomena that are remarkably similar to speech acquisition in humans, including sensitive phases that depend on external models for acquisition of sounds, and that requirement for intact auditory feedback during development but not (in general) in adulthood. Thus, the mechanistic analysis of song learning can increase knowledge of the specific animal test system, can inform larger neuroethological theories of behavior, and can shed light on specific pathological human conditions such as congenital deafness. Song learning endows neurons in "song system" nuclei such as HVc with specificity for acoustic parameters of the individual bird's autogenous song(AS). These properties result from learned sensorimotor interactions in the song system. HIVc gives rise to a motor pathway and to a pathway involved in song learning. This proposal is to analyze the functional interactions between the HVc projection neurons giving rise to the two pathways. It is predicted that lesions of the X-projecting HVc neurons (HVc-Xn) will result in stabilization of an abnormal song, that lesions of the RA-projecting HVc neurons (HVc-RAn) will result in recovery of the normal song, that different subdivisions of HVc will have different roles in song production and differences in morphology, physiology and recovery, and that interactions between HVc-RAn and HVc-Xn are necessary to establish AS selectivity. In Exp. I, the roles of Hvc-Xn and HVc-RAn in song production will be tested by assessing changes in song production over extended periods of time following selective laser photoablation of projection neurons retrogradely labeled with the phototoxic dye eosin. Songs will be analyzed by newly developed algorithms. Control experiments will establish standardized treatments. The role of auditory feedback in song stabization and song recovery will then by assessed by eliminating auditory feedback or by provide altered auditory feedback. The potential role of adult neurogenesis in song stabilization and song recovery will be assessed using the standardized treatments combined with histological techniques to identify the disposition of new HVcRAn that are incorporated in circuits subsequent to the photolesions. In Exp. 2 and 3, fluorescently-labeled HVc projection neurons will be visualized in vivo. In Exp. 2 extracellular recordings will measure spontaneous activity, selectivity/specificity to AS, synchronous activity, and changes in those properties in each subdivision immediately after and in days following lesions restricted to HVc-RAn and/or HVc-Xn of a single subdivision. In the third experiment, the morphology of intracellularly labeled neurons in each subdivision of HVc will be characterized to help establish an anatomical basis for the functional differences.